In his first-ever interview, the sole Australian survivor of the Waco siege has revealed to 60 Minutes what really took place during the 51-day stand-off, and why he believes crazed cult leader David Koresh will one day return to Earth as the son of God.

MP shocked by daughter's gay relationship

The Queensland Liberal backbencher is a committed Catholic who goes to church most Sundays and even joined a monastery as a teenager.

"About three years ago our daughter told my wife and I that she was attracted to women - that she had a girlfriend," Mr Wallace told parliament on Tuesday, during debate on legalising same-sex marriage.

"My wife and I were shocked. Probably more me than my wife. I didn't know what to say."

Mr Wallace believed the Catholic teaching that marriage was between a man and a woman.

"Homosexuality went against what I had been taught to believe for many years. How could this be happening? How could this be happening to me, to our family?," he asked.

His daughter Caroline struggled with mental illness and eating disorders throughout her teenage years.

"She has a terrific job and a wonderful partner who our family love very much."

Caroline offered her father a poignant parallel during their initial discussion that proved to be an epiphany for him.

"She said, 'Dad, in the years to come, my generation will look back and judge your generation about how you deal with the issue of homosexuality'," he told MPs.

"In the same way your generation considered your parents' generation in the way they dealt with our indigenous people."

Labor frontbencher Linda Burney choked back tears while explaining she supported marriage equality, saying her 33-year-old son Binni, who died in October, was gay.

"I have seen first-hand the confusion, anxiety and pain that many of our young people experience struggling with their sexuality," she said.

Labor MP Cathy O'Toole's daughter Louise knew quite early on that she was gay, but did not act on her feelings until her early 20s out of fear. She struggled with depression and anxiety throughout her teens.

Next year, Louise will marry her partner Cat.

"As a mother, all I have ever wanted for all of my children is for them to be happy, to belong, and to be accepted them for who they are," Ms O'Toole told parliament.

Mr Wallace will support same-sex marriage but wants to see greater religious protections in the bill.

Nationals MP Andrew Broad, an opponent of same-sex marriage, will vote in favour of the legislation in line with his western Victorian electorate but also wants to see greater religious protections.